How do you combine a sculpt mesh with primitives for 3d printing?
You can use the Boolean Modifier to cut out the tube from your model, and combine it with scaffolding.
That said, scaffolding is usually added when slicing the 3D finished model in your 3D printing software. I'm not sure if you will get good results if you try to do this in the modeling step.
To add thickness to a sheet-like object, you can use the Solidify Modifier.